Page:EB1911 - Volume 01.djvu/554

 in the transformation of the “white” of an egg on boiling. Albumins are generally detected by taking advantage of this property, or of certain colour changes. The reagents in common use are: Millon’s reagent, a solution of mercuric nitrate containing nitrous acid, this gives a violet-red coloration; nitric acid, which gives a yellow colour, turning to gold when treated with ammonia (xanthoproteic reaction); fuming sulphuric acid, which gives violet solutions; and caustic potash and copper sulphate, which, on warming, gives a red to violet coloration (biuret reaction).

Boiling with dilute mineral acids, or baryta water, decomposes albumins into carbon dioxide, ammonia and fatty amino- and other acids. These decomposition products include: glycocoll or aminoacetic acid, NH2CH2COOH, alanine or aminopropionic acid, CH3.CH(NH2).COOH, -aminobutyric acid, -aminovalerianic acid, leucin or isobutyl--aminoacetic acid, (CH3)2CH.CH2.CH(NH2).COOH, isoleucin, probably -aminocaproic acid, serin or -amino--hydroxypropionic acid, HO.CH2.CH(NH2).COOH, aspartic acid or aminosuccinic acid, HOOC.CH2.CH(NH2).COOH, glutaminic acid or -amino--glutaric acid, HOOC.(CH2)2.CH(NH2).COOH, diaminoacetic acid, --diaminopropionic acid, lysin. or --diamino-n-caproic acid, NH2(CH2)4.CH(NH2).COOH, arginin or guanidine--amino-n-valerianic acid, (NH)(NH2)C.NH.(CH2)3.CH(NH2).COOH, ornithin or -diamino valerianic acid, NH2.(CH2)3.CH(NH2).COOH, histidin or -amino--imidazol-propionic acid, proline or -pyrrolidin carboxylic acid, , hydroxyproline, phenyl alanine or phenyl--aminopropionic acid, C6H5.CH2.CH(NH2).COOH, tyrosine or p-hydroxyphenyl--aminopropionic acid, phenyl ethylamine, p-hydroxyphenyl ethylamine, tryptophane or indol aminopropionic acid, A. cystin (protein-cystin) or -amino--thioglyceric acid “disulphide,” (S.CH2.CH(NH2).COOH)2, B. cystin (stone-cystin), or -thio--aminoglyceric acid “disulphide,” (NH2.CH2.CH:S.COOH)2. This list is not exhaustive; other products are given in Gustav Mann, Chemistry of the Proteids (1906), to which reference should be made for a complete account of this class of compounds.

The complexity of composition militates in a great measure against a rational classification of albumins by purely chemical considerations. Such classifications have been attempted by A. Kossel and by W. Kuhne and E. P. Pick; but in the present state of our knowledge, however, the older classification of E. Dreschel and F. Hoppe-Seyler, based primarily on solubilities and distribution, may be conveniently retained. This classification is with certain modifications as follows:—  Albumins proper: characterized by having colloidal solutions.  Albumins: serum-albumin, egg-albumin, lact-albumin. Globulins: serum-globulin, egg-globulin, lacto-globulin, cell-globulins. Plant-globulins and plant-vitellines. Fibrinogen. Myosin. Phosphorus containing albumins (nucleo-albumins), caseins, vitellines, nucleo-albumins of the cell-protoplasm, mucoid nucleo-albumins. <li>Histones.</li> <li> Protamines.</li> </ol></li> <li>Transformation products of the albumins proper. <ol> <li>Acid-albumins, alkali albuminates.</li> <li>Albumoses, peptones and peptides.</li> <li>Halogen-albumins, oxyprotein, oxyprotsulphonic acid, &c.</li> </ol></li> <li>Proteids. <ol> <li>Nucleo-proteids.</li> <li>Haemoglobin and allied substances.</li> <li>Glyco-proteids, mucins, mucoids, helico-proteid.</li> </ol></li> <li>Albuminoids. <ol> <li>Collagen.</li> <li>Keratin.</li> <li>Elastin.</li> <li>Fibroin.</li> <li>Spongin, &c.</li> <li>Amyloid.</li> <li>Albumoid.</li> <li>Colouring matters derived from albumin.</li> </ol></li></ol>

Albumins proper.—Albumins (as classified above) are soluble in water, dilute acids and alkalies, and in saturated neutral salt solutions; they are coagulated by heat. “Serum- albumin,” or “blood-albumin,” possibly C450H720N116S6O140, occurs in blood-serum, lymph, chyle, milk, &c.; its coagulation temperature is about 67°. It differs from egg-albumin in its specific rotation (−57° to −64°), and in being slowly coagulated by alcohol and ether. Egg-albumin is the chief constituent of the white of egg; this fluid also contains a globulin and a mucoid. It coagulates at about 56°, and its specific rotation is −30·70°. “Lact-albumin” occurs in all kinds of milk. The globulins are insoluble in water and in dilute acids, but soluble in alkalies and in neutral salt solutions; these solutions are coagulated on boiling. “Serum-globulin,” also termed globulin or fibrino-plastic globulin, paraglobulin and paraglobin, occurs in blood serum; “cell-globulins” occur in many organs—liver, kidneys, pancreas and the thyroid gland, also in muscle-plasma; “crystalline,” a globulin occurring in two forms and, is found in the lens of the eye; “egg-globulin” and “lacto-globulin” occur respectively in the white of egg and in milk. Plant albumins or phyto-albumins have been chiefly investigated in the case of those occurring in seeds; most are globulins, insoluble in pure water, but soluble in salt solutions; “edestin,” a globulin of this class, is very widely distributed. Other varieties or classes of these compounds are: plant caseins, phyto-vitellines, legumins and conglutins. Fibrinogen occurs in the blood plasma, and is changed by a ferment into fibrin, to which the clotting of blood is due. Fibrinogen is insoluble in water, but soluble in salt solutions; it has three different coagulation temperatures, 56°, 67°, 75°. Fibrin, produced from fibrinogen by a ferment, is a jelly-like substance, coagulable by heat, alcohol, &c. The muscle-albumins include “myosin” or paramyosinogen, a globulin, which by coagulation induces rigor mortis, and the closely related “myosinogen” or myogen; myoglobulin and myoalbumin are also found in muscles. The nucleo-albumins or phospho-globulins are insoluble in water and acids, but soluble in alkalies, and have an acid reaction. “Caseinogen” (after W. D. Halliburton) is the chief albumin of milk; its composition varies with the animal. It is insoluble in water, while its salts are readily soluble. “Eucasein” is the ammonium salt; “nutrose” and “plasmon” are sodium salts. By the rennet ferment caseinogen is converted into casein, a substance resembling caseinogen in being soluble in water, but differing in having an insoluble calcium salt. The formation of casein involves the curdling of milk. Other phosphoglobulins are vitelline, found in the yolk of hens' eggs, and ichthulin, found in the eggs of fish. Histones are a class of albumins soluble in water and acids, but essentially basic in character; hence they are precipitated by alkalies. It is remarkable that many histones are soluble in an excess of alkali. They do not exist in a free state, but in combination with a “prosthetic group” (after A. Kossel) they give rise to important cell constituents—haemoglobin, nucleo-proteids, &c. “Thymus histone” occurs in the thymus gland; globin occurs in combination as haemoglobin; other histones have been extracted from the red blood corpuscles of the goose and the testes of fishes and other animals. The protamines are a well-characterized class of albumins found in the ripe spermatozoa of fishes.

Albumoses and Peptones.—The primary products of the dissociation of albumins are the albumoses, characterized by not being coagulable by heat, more soluble than the albumins, having a far less complex composition, and capable of being “salted